--orders: route several connection orders in parallel, keep the best

The default order is always one of the candidates, so the result is
never worse than a plain run; ties go to the earlier candidate.

tests/route/test_orders.py

@@ -0,0 +1,44 @@
+"""--orders: route several connection orders and keep the best."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from freeroute.cli import main
+from freeroute.route.orders import best_of_orders, candidate_seeds, route_once, score
+
+FIXTURES = Path(__file__).resolve().parent.parent / "dsn" / "fixtures"
+# Greedy (no rip-up), the default order strands a net here and seeds 1 to 3 do not.
+RIPUP = (FIXTURES / "ripup_needed.dsn").read_text()
+GREEDY = {"rip_up": False}
+
+
+def test_the_default_order_is_always_a_candidate_without_a_seed():
+    assert candidate_seeds(1) == [None]
+    assert candidate_seeds(4) == [None, 0, 1, 2]
+    assert candidate_seeds(3, seed=7) == [7, 8, 9]
+
+
+def test_keeps_the_order_that_leaves_the_fewest_unrouted():
+    assert score(route_once(RIPUP, "grid", GREEDY, None)) == (1, 0)
+    result, kept, tried = best_of_orders(RIPUP, "grid", GREEDY, orders=3, jobs=2)
+    assert kept == 1
+    assert score(result) == (0, 0)
+    assert tried == [(None, (1, 0)), (0, (1, 0)), (1, (0, 0))]
+
+
+def test_ties_go_to_the_earlier_candidate():
+    # every order routes this board fully, so the default order is kept
+    _, kept, tried = best_of_orders(RIPUP, "grid", {}, orders=3, jobs=3)
+    assert kept is None
+    assert all(s == (0, 0) for _, s in tried)
+
+
+def test_cli_orders_writes_the_best_session(tmp_path, capsys):
+    out = tmp_path / "best.ses"
+    rc = main([str(FIXTURES / "ripup_needed.dsn"), "--no-rip-up", "--orders", "3", "-do", str(out)])
+    assert rc == 0
+    err = capsys.readouterr().err
+    assert "tried 3 orders, kept --seed 1" in err
+    assert "not routed" not in err
+    assert out.read_text().startswith("(session")